Conducting polymers, or polymers that have conjugation in their backbone, can be used in many commercial applications including for example OLEDs, PLEDs, photovoltaic cells, transistors, sensors, organic solar cells, and the like. They are ideal for these uses since they are easily tunable and have their ability to be produced through a solution process leads to low-cost fabrication.
In many cases, commercial applications compel that these polymers be free of metallic impurities. For example, nickel, palladium, ruthenium and zinc represent typical impurities that can be present in conjugated polymers. Trace amounts of these metals are detrimental to organic electronic device performance. Conducting polymers can aggregate and trap these impurities making them difficult to remove. Palladium is the most common catalyst used in preparing conjugated polymers and may stubbornly persist in the material despite careful workup and extensive efforts to purify the polymers.
Several methods have been commonly used to purify conjugated polymers including precipitation, preparatory scale gel permeation chromatography, column chromatography using silica gel and Soxhlet extraction. Each of these methods has drawbacks and limitations in association with conjugated polymer purification, including amount of time and solvent required, scalability of the process and lack of reduction in impurities and oligomers.
One commonly used method is precipitation of the polymers, or adding a solution of the polymer to a “bad” solvent to generate the polymer as a precipitate. While this method is quick and does not generate large amounts of solvent waste, precipitation is largely ineffective at removing impurities and low molecular weight oligomers.
Preparatory scale gel permeation chromatography is a size exclusion chromatography method, thus this method can easily be used to remove low molecular weight oligomers from the polymer. However, only small amounts of polymer (<15 mg) can be purified at one time. Silica gel chromatography has also been used in effort to purify conjugated polymers; however it is unclear how effective this technique is at reducing impurities. In addition, the polymers often adhere to the silica gel resulting in a large loss of product. Each of these chromatography methods generate large amounts of solvent waste, does not fully remove metal impurities, and are not practical for industrial applications.
Another popular technique for purifying conjugated polymers is the use of a Soxhlet extractor, where the polymer is washed repeatedly with a series of hot solvents. Solvents that will not dissolve the polymer such as methanol and acetone are used in effort to remove impurities, and solvents such as hexanes are used to dissolve low molecular weight oligomers. Finally, a solvent such as chloroform is used to dissolve the polymer. A Soxhlet extraction involves a large investment of time to thoroughly wash the polymers, generally 24-48 hours for each type of solvent wash. Soxhlet extraction is not an effective method for fully removing metal impurities trapped in the polymers.
A general need exists to find a versatile, inexpensive, convenient, commercially attractive method to purify conjugated polymers.